1. Field of the Invention
This invention relates to photoacoustic spectrometers and, more specifically, to such spectrometers with improved sample analysis capability.
2. Prior Art
Photoacoustic spectrometry has become a popular non-destructive analysis method. It relies upon the fact that various materials, when illuminated, convert the absorbed light into heat at various rates and at various efficiencies. The absorption spectrum is peculiar to each substance. Photoacoustic spectrometry quantifies the heat generation by measuring, directly or indirectly, the thermal expansion of the gas surrounding the sample when the sample is illuminated. One method (and apparatus) for achieving this measurement is shown in co-pending application Ser. No. 55,272 invented by Atsuo Watanabe, et al. and filed July 6, 1979 and assigned to the same assignee as this application. In that apparatus, the rate of flow of gas from an illuminated sample chamber to an un-illuminated reference chamber is measured by means of a flow-meter which produces a periodically interrupted d.c. output corresponding to the pattern of interrupting the illumination to the sample cell. If two such units are utilized and a known material is placed in one sample chamber and an unknown in the other sample chamber, both chambers being illuminated by chopped light from a common monochromatic light source, a relatively accurate analysis of the composition of the unknown sample is possible. However, time variations in light intensity and chopping frequency produce spurious phenomena or "noise" in the test results and inaccuracy in the related analysis. One factor of great importance in achieving accuracy of analysis is the time delay in the sample-response signal relative to the illuminating signal.
Prior art devices did not have accurate means for determining such delay--referred to as ".theta.," a phase-angle. Also, as the frequency of light chopping is varied as part of the analysis technique, phase is not necessarily preserved and the accuracy of ".theta." is adversely affected.
Many conventional photoacoustic spectrometers can produce both in-phase and quadrature outputs, but such dual-phase-output spectrometers are no more accurate than single-phase-output spectrometers as far as ".theta." is concerned. In such dual-phase-output spectrometers the "in-phase" output signal is produced by synchronous rectification in a phase condition where the signal is at is maximum. The quadrature output is produced by a 90.degree. phase delay. Producing an accurate 90.degree. phase shift at these low chopping frequencies is very difficult and analysis inaccuracies result.
Therefore, it is an object of the present invention to overcome the general disadvantages set forth hereinbefore.
It is a further object of the present invention to provide a photoacoustic spectrometer with enhanced sample analysis capabilities.
It is a still further object of this invention to provide a photoacoustic spectrometer which is free of synchronous rectification requirements.